Quicktime aspect ratio problem

I've just upgraded my Macbook Pro to Maverick. My Quicktime Player has concurrently started playing my videos in the wrong aspect ratio. Are these related? I shoot MP4 videos that are 1980x1080 px size. They played just fine before my upgrade. Now they are stretched and most of my view options are not available.
Any help or advice would be appreciated.

Download "VLC"... way better than quicktime.
VideoLAN - Official page for VLC media player, the Open Source video framework!

While that is a valid alternative it doesn't address the problem.
Your resolution may have changed with the upgrade. Open System Preferences and click on Displays. Hit the Scaled button and check the resolutions. If it's different than what it was or you just want to experiment you can change it there.